The instant invention pertains to apparatus for skidding a load on a support and more particularly to such apparatus which incorporate rollers to support at least part of the load to facilitate skidding.
When it is desired to move a load on a support, for example a well-drilling rig on a platform, the load can simply be skidded across the support. In the case of a drilling rig, this is conventionally achieved by the use of hydraulic cylinders and pistons which may be connected to the rig and to selected points on the platform to slide the rig along the platform. U.S. Pat. No. 3,486,737 to Campbell, assigned to the assignee of the instant application, discloses an apparatus for skidding a load on a support. In Campbell, an hydraulic ram has its cylinder end connected to a drilling rig and its rod connected to a roller which is positioned beneath the rig. A friction-actuated lifting device is operable between the roller and the platform floor to lift the roller as the ram extends. Such lifting transfers at least part of the load to the roller to facilitate skidding of the rig.
The design disclosed in the Campbell patent represents a clear advance over prior art at the time. Subsequent to Campbell, further improvements in skidding apparatus were made in which the friction-actuated lifting device was replaced with a pair of lifting rollers which operate on a cam surface. A skidding roller, which serves the same function as the roller in Campbell, is sandwiched between the lifting rollers and all of the rollers are rotatable about a common axis. As the outer roller pair is forced up a cam surface by means of an hydraulic ram, at least some of the weight of the rig is shifted to the middle roller to facilitate rig skidding.
The roller skidding method and apparatus subsequent to Campbell suffers from drawbacks which are related to the structure of the typical platform upon which a drilling rig is to be skidded. The usual platform includes a pair of I-beams known as capping beams, each of which has a vertical web between upper and lower horizontal flanges. A skidding apparatus is positioned beneath each side of the rig over each capping beam. The skidding apparatus, and hence the weight of the rig, is supported by the top flange of the capping beams. The lifting rollers load the capping beam along its outer flange edges which is the least desirable way of loading a beam, the most desirable being loading at the center of the flange over the web. In some instances, the flange of the capping beam is so narrow that the lifting rollers extend over the edges of the capping beam flange thereby preventing its use due to potential instability.
Proper loading of the capping beam has become more important in that rigs weighing as much as a million pounds may be skidded on a platform.
The instant invention comprises an improved method and apparatus which includes, for each lifting roller, a support beam therebeneath having its vertical web in alignment with the vertical axis of the roller. For each skidding roller, a load beam having a vertical web is mounted on the underside of the load. To transmit the weight of the load to the skidding roller the load beam web is in vertical alignment with the vertical axis of the skidding roller. As the rig is skidded, the vertical axes of the rollers are maintained in alignment with the vertical axes of each roller's associated web.
It is an advantage of the instant invention to provide an improved method and apparatus for skidding a load on a support which optimally loads I-beams in the support and on the underside of the load.
It is another advantage of the instant invention to provide such an apparatus which may be utilized with support I-beams having a narrow flange.